Wafer processing method of uniting a wafer and a ring frame using a polyolefin sheet

ABSTRACT

A wafer processing method includes a polyolefin sheet providing step of positioning a wafer in an inside opening of a ring frame and providing a polyolefin sheet on a back side or a front side of the wafer and on a back side of the ring frame, a uniting step of heating the polyolefin sheet as applying a pressure to the polyolefin sheet to thereby unite the wafer and the ring frame through the polyolefin sheet by thermocompression bonding, a dividing step of applying a laser beam to the wafer to form modified layers in the wafer, thereby dividing the wafer into individual device chips, and a pickup step of picking up each device chip from the polyolefin sheet.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer processing method for dividinga wafer along a plurality of division lines to obtain a plurality ofindividual device chips, the division lines being formed on the frontside of the wafer to thereby define a plurality of separate regionswhere a plurality of devices are individually formed.

Description of the Related Art

In a fabrication process for device chips to be used in electronicequipment such as mobile phones and personal computers, a plurality ofcrossing division lines (streets) are first set on the front side of awafer formed of a semiconductor, for example, thereby defining aplurality of separate regions on the front side of the wafer. In eachseparate region, a device such as an integrated circuit (IC), alarge-scale integrated circuit (LSI), and a light emitting diode (LED)is next formed. Thereafter, a ring frame having an inside opening isprepared, in which an adhesive tape called a dicing tape is previouslyattached in its peripheral portion to the ring frame so as to close theinside opening of the ring frame. Thereafter, a central portion of theadhesive tape is attached to the back side or the front side of thewafer such that the wafer is accommodated in the inside opening of thering frame. In this manner, the wafer, the adhesive tape, and the ringframe are united together to form a frame unit. Thereafter, the waferincluded in this frame unit is processed to be divided along eachdivision line, thereby obtaining a plurality of individual device chipsincluding the respective devices.

For example, a laser processing apparatus is used to divide the wafer.The laser processing apparatus includes a chuck table for holding thewafer through the adhesive tape and a laser processing unit for focusinga laser beam inside the wafer held on the chuck table, the laser beamhaving a transmission wavelength to the wafer. In dividing the wafer byusing this laser processing apparatus, the frame unit is placed on thechuck table, and the wafer is held through the adhesive tape on theupper surface of the chuck table. In this condition, the chuck table andthe laser processing unit are relatively moved in a direction parallelto the upper surface of the chuck table. At the same time, the laserbeam is applied from the laser processing unit to the wafer along eachdivision line. When the laser beam is focused inside the wafer, amodified layer serving as a division start point is formed in the waferalong each division line (see Japanese Patent No. 3408805).

Thereafter, the frame unit is transferred from the laser processingapparatus to another apparatus, and the adhesive tape is expanded in aradially outward direction, so that the wafer is divided into individualdevice chips. When the device chips thus formed are picked up from theadhesive tape, ultraviolet light is applied to the adhesive tape inadvance, for example, to thereby reduce the adhesion of the adhesivetape. Thereafter, each device chip is picked up from the adhesive tape.As a processing apparatus capable of producing the device chips withhigh efficiency, there is a processing apparatus capable of continuouslyperforming the operation for dividing the wafer and the operation forapplying ultraviolet light to the adhesive tape (see Japanese Patent No.3076179, for example).

SUMMARY OF THE INVENTION

The adhesive tape includes a base layer formed from a polyvinyl chloridesheet, for example, and an adhesive layer formed on the base layer. Inthe laser processing apparatus, the laser beam is focused inside thewafer in order to form a modified layer which serves as a division startpoint, inside the wafer, and part of leaked light of the laser beamreaches the adhesive layer of the adhesive tape. As a result, theadhesive layer of the adhesive tape attached to the back side or thefront side of the wafer is melted by the heat due to the application ofthe laser beam to the wafer at the position below or around eachdivision groove formed in the wafer, and a part of the adhesive layermelted is fixed to the back side or the front side of each device chipobtained from the wafer. In this case, in the step of picking up eachdevice chip from the adhesive tape, ultraviolet light is applied to theadhesive tape to reduce the adhesion of the adhesive tape. However, thepart of the adhesive layer melted and fixed to the back side or thefront side of each device chip attached to the adhesive tape is yet lefton the back side or the front side of each device chip picked up fromthe adhesive tape. As a result, the quality of each device chip isdegraded.

It is therefore an object of the present invention to provide a waferprocessing method which can prevent the adherence of the adhesive layerto the back side or the front side of each device chip obtained from awafer, thereby suppressing a degradation in quality of each device chipdue to the adherence of the adhesive layer.

In accordance with an aspect of the present invention, there is provideda wafer processing method for dividing a wafer along a plurality ofdivision lines to obtain a plurality of individual device chips, thedivision lines being formed on a front side of the wafer to therebydefine a plurality of separate regions where a plurality of devices areindividually formed. The wafer processing method includes a ring framepreparing step of preparing a ring frame having an inside opening foraccommodating the wafer, a polyolefin sheet providing step ofpositioning the wafer in the inside opening of the ring frame andproviding a polyolefin sheet on a back side or the front side of thewafer and on a back side of the ring frame, a uniting step of heatingthe polyolefin sheet as applying a pressure to the polyolefin sheetafter performing the polyolefin sheet providing step, thereby unitingthe wafer and the ring frame through the polyolefin sheet bythermocompression bonding to form a frame unit in a condition where thefront side of the wafer and the front side of the ring frame are exposedupward, a dividing step of positioning a focal point of a laser beaminside the wafer, the laser beam having a transmission wavelength to thewafer, and applying the laser beam to the wafer along each divisionline, thereby forming a modified layer in the wafer along each divisionline to divide the wafer into the individual device chips, afterperforming the uniting step, and a pickup step of picking up each devicechip from the polyolefin sheet after performing the dividing step.

Preferably, the uniting step includes a step of applying infrared lightto the polyolefin sheet, thereby performing the thermocompressionbonding.

Preferably, the polyolefin sheet is larger in size than the ring frame,and the uniting step includes an additional step of cutting thepolyolefin sheet after heating the polyolefin sheet, thereby removing apart of the polyolefin sheet outside an outer circumference of the ringframe.

Preferably, the pickup step includes a step of expanding the polyolefinsheet to thereby increase a spacing between any adjacent ones of thedevice chips, and each device chip is pushed up through the polyolefinsheet.

Preferably, the polyolefin sheet is formed of a material selected fromthe group consisting of polyethylene, polypropylene, and polystyrene.

In the case that the polyolefin sheet is formed of polyethylene, thepolyolefin sheet is preferably heated in the range of 120° C. to 140° C.in the uniting step. In the case that the polyolefin sheet is formed ofpolypropylene, the polyolefin sheet is preferably heated in the range of160° C. to 180° C. in the uniting step. In the case that the polyolefinsheet is formed of polystyrene, the polyolefin sheet is preferablyheated in the range of 220° C. to 240° C. in the uniting step.

Preferably, the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.

In the wafer processing method according to a preferred embodiment ofthe present invention, the wafer and the ring frame are united by usingthe polyolefin sheet having no adhesive layer in place of an adhesivetape having an adhesive layer, thereby forming the frame unit composedof the wafer, the ring frame, and the polyolefin sheet united together.The uniting step of uniting the wafer and the ring frame through thepolyolefin sheet is realized by thermocompression bonding. Afterperforming the uniting step, a laser beam having a transmissionwavelength to the wafer is applied to the wafer to thereby form amodified layer inside the wafer along each division line, so that thewafer is divided along each division line to obtain individual devicechips attached to the polyolefin sheet. Thereafter, each device chip isthen picked up from the polyolefin sheet. Each device chip picked up isnext mounted on a predetermined mounting substrate or the like.

In forming a modified layer inside the wafer, leaked light of the laserbeam reaches the polyolefin sheet. However, since the polyolefin sheethas no adhesive layer, there is no problem that the adhesive layer maybe melted to be fixed to the back side or the front side of each devicechip. That is, according to one aspect of the present invention, theframe unit can be formed by using the polyolefin sheet having noadhesive layer, so that an adhesive tape having an adhesive layer is notrequired. As a result, it is possible to prevent the problem that thequality of each device chip is degraded by the adherence of the adhesivelayer to each device chip.

Thus, the wafer processing method according to one aspect of the presentinvention can exhibit the effect that the adhesive layer does not adhereto the back side or the front side of each device chip obtained from thewafer, thereby suppressing a degradation in quality of each device chipdue to the adherence of the adhesive layer.

The above and other objects, features, and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings depicting a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of the front side of a wafer;

FIG. 1B is a schematic perspective view of the back side of the wafer;

FIG. 2 is a schematic perspective view depicting a manner of positioningthe wafer and a ring frame on a holding surface of a chuck table;

FIG. 3 is a schematic perspective view depicting a polyolefin sheetproviding step;

FIG. 4 is a schematic perspective view depicting a uniting step;

FIG. 5 is a schematic perspective view depicting a modification of theuniting step;

FIG. 6 is a schematic perspective view depicting another modification ofthe uniting step;

FIG. 7A is a schematic perspective view depicting a manner of cuttingthe polyolefin sheet after performing the uniting step;

FIG. 7B is a schematic perspective view of a frame unit formed byperforming the step depicted in FIG. 7A;

FIG. 8A is a schematic perspective view depicting a dividing step;

FIG. 8B is a schematic sectional view depicting the dividing step;

FIG. 9 is a schematic perspective view depicting a manner of loading theframe unit to a pickup apparatus after performing the dividing step;

FIG. 10A is a schematic sectional view depicting a standby conditionwhere the frame unit is fixed to a frame support table set at an initialposition in a pickup step using the pickup apparatus depicted in FIG. 9;and

FIG. 10B is a schematic sectional view depicting a working conditionwhere the frame support table holding the frame unit with the polyolefinsheet is lowered to expand the polyolefin sheet in the pickup step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be describedwith reference to the attached drawings. There will first be described awafer to be processed by a wafer processing method according to thispreferred embodiment. FIG. 1A is a schematic perspective view of thefront side of a wafer 1. FIG. 1B is a schematic perspective view of theback side of the wafer. The wafer 1 is a substantially disc-shapedsubstrate formed of a material such as silicon (Si), silicon carbide(SiC), gallium nitride (GaN), and gallium arsenide (GaAs). The wafer 1may be formed of any other semiconductor materials. Further, the wafer 1may be formed of a material such as sapphire, glass, and quartz.Examples of the glass include alkaline glass, nonalkaline glass, sodalime glass, lead glass, borosilicate glass, and silica glass. The wafer1 has a front side 1 a and a back side 1 b. A plurality of crossingdivision lines 3 are formed on the front side 1 a of the wafer 1 tothereby define a plurality of respective separate regions where aplurality of devices 5 such as ICs, LSIs, and LEDs are formed. Thecrossing division lines 3 are composed of a plurality of paralleldivision lines 3 extending in a first direction and a plurality ofparallel division lines 3 extending in a second direction perpendicularto the first direction. In the processing method for the wafer 1according to this preferred embodiment, a plurality of modified layersare formed inside the wafer along the respective crossing division lines3 in the wafer 1, thereby dividing the wafer 1 into a plurality ofindividual device chips each including the device 5 with the modifiedlayers as division start points.

In forming a modified layer in the wafer 1, a laser beam having atransmission wavelength to the wafer 1 is applied to the wafer 1 alongeach of the division lines 3, thereby focusing the laser beam inside thewafer 1. At this time, the laser beam may be applied to the wafer 1 fromthe front side 1 a depicted in FIG. 1A or from the back side 1 bdepicted in FIG. 1B. Note that, in a case where the laser beam may beapplied to the wafer 1 from the back side 1 b, alignment means includingan infrared camera is used to detect each of the division lines 3 fromthe front side 1 a through the wafer 1, so that the laser beam isapplied to the wafer 1 along each of the division lines 3.

The laser processing to form a modified layer in the wafer 1 isperformed by using a laser processing apparatus 12 (see FIG. 8). Priorto loading the wafer 1 into the laser processing apparatus 12, the wafer1 is united with a polyolefin sheet 9 (see FIG. 3) and a ring frame 7(see FIG. 2) to thereby form a frame unit 11 (see FIG. 8). Thus, thewafer 1 is loaded in the form of the frame unit 11 into the laserprocessing apparatus 12 and then processed by the laser processingapparatus 12. Then, the polyolefin sheet 9 is expanded to divide thewafer 1, thereby obtaining the individual device chips, in which eachdevice chip is supported to the polyolefin sheet 9. Thereafter, thepolyolefin sheet 9 is further expanded to thereby increase the spacingbetween any adjacent ones of the device chips. Thereafter, each devicechip is picked up by using a pickup apparatus. The ring frame 7 isformed of a rigid material such as metal, and it has a circular insideopening 7 a having a diameter larger than that of the wafer 1. Theoutside shape of the ring frame 7 is substantially circular. The ringframe 7 has a front side 7 b and a back side 7 c. In forming the frameunit, the wafer 1 is accommodated in the inside opening 7 a of the ringframe 7 and positioned in such a manner that the center of the wafer 1substantially coincides with the center of the inside opening 7 a.

The polyolefin sheet 9 is a flexible (expandable) resin sheet, and ithas a flat front side and a flat back side. The polyolefin sheet 9 is acircular sheet having a diameter larger than the outer diameter of thering frame 7. The polyolefin sheet 9 has no adhesive layer. Thepolyolefin sheet 9 is a sheet of a polymer (polyolefin) synthesized bypolymerizing alkene as a monomer. Examples of the polyolefin sheet 9include a polyethylene sheet, polypropylene sheet, and polystyrenesheet. The polyolefin sheet 9 is transparent or translucent to visiblelight. As a modification, the polyolefin sheet 9 may be opaque. Sincethe polyolefin sheet 9 has no adhesive property, it cannot be attachedto the wafer 1 and the ring frame 7 at room temperature. However, thepolyolefin sheet 9 is a thermoplastic sheet, so that, when thepolyolefin sheet 9 is heated to a temperature near its melting pointunder a predetermined pressure in a condition where the polyolefin sheet9 is in contact with the wafer 1 and the ring frame 7, the polyolefinsheet 9 is partially melted and thereby bonded to the wafer 1 and thering frame 7. That is, by applying heat and pressure to the polyolefinsheet 9 in the condition where the polyolefin sheet 9 is in contact withthe wafer 1 and the ring frame 7, the polyolefin sheet 9 can be bondedto the wafer 1 and the ring frame 7. Thusly, in the processing methodfor the wafer 1 according to this preferred embodiment, all of the wafer1, the ring frame 7, and the polyolefin sheet 9 are united bythermocompression bonding as mentioned above, thereby forming the frameunit.

The steps of the processing method for the wafer 1 according to thispreferred embodiment will now be described. Prior to uniting the wafer1, the polyolefin sheet 9, and the ring frame 7, a polyolefin sheetproviding step is performed by using a chuck table 2 having a holdingsurface 2 a depicted in FIG. 2. FIG. 2 is a schematic perspective viewdepicting a manner of positioning the wafer 1 and the ring frame 7 onthe holding surface 2 a of the chuck table 2. That is, the polyolefinsheet providing step is performed on the holding surface 2 a of thechuck table 2 as depicted in FIG. 2. The chuck table 2 has a circularporous member having a diameter larger than the outer diameter of thering frame 7. The porous member constitutes a central upper portion ofthe chuck table 2. The porous member has an upper surface functioning asthe holding surface 2 a of the chuck table 2. A suction passage (notdepicted) is formed in the chuck table 2, in which one end of thesuction passage is connected to the porous member. Further, a vacuumsource 2 b (see FIG. 3) is connected to the other end of the suctionpassage. The suction passage is provided with a selector 2 c (see FIG.3) for switching between an ON condition and an OFF condition. When theON condition is established by the selector 2 c, a vacuum produced bythe vacuum source 2 b is applied to a workpiece placed on the holdingsurface 2 a of the chuck table 2, thereby holding the workpiece on thechuck table 2 under suction.

In the polyolefin sheet providing step, the wafer 1 and the ring frame 7are first placed on the holding surface 2 a of the chuck table 2 asdepicted in FIG. 2. In this condition, the wafer 1 is positioned in theinside opening 7 a of the ring frame 7. At this time, taking it intoconsideration which of the front side 1 a or the back side 1 b is to bean application surface to be applied by the laser beam in the laterdividing step, an orientation of the wafer 1 is selected. For example,in a case where the front side 1 a is selected as the applicationsurface, the front side 1 a is orientated downward. Alternatively, forexample, in a case where the back side 1 b is selected as theapplication surface, the back side 1 b is orientated downward. A waferprocessing method according to the preferred embodiment will bedescribed below taking the case where the front side 1 a is selected asthe application surface of the laser beam as an example. However, theorientation of the wafer 1 is not limited to this.

After the wafer 1 and the frame 7 are placed on the holding surface 2 aof the chuck table 2, the polyolefin sheet 9 is provided on the backside 1 b (or the front side 1 a) (upper surface) of the wafer 1 and onthe back side 7 c (upper surface) of the ring frame 7. FIG. 3 is aschematic perspective view depicting a manner of providing thepolyolefin sheet 9 on the wafer 1 and the ring frame 7. That is, asdepicted in FIG. 3, the polyolefin sheet 9 is provided so as to fullycover the wafer 1 and the ring frame 7. In the polyolefin sheetproviding step, the diameter of the polyolefin sheet 9 is set largerthan the diameter of the holding surface 2 a of the chuck table 2.Unless the diameter of the polyolefin sheet 9 is larger than thediameter of the holding surface 2 a, there may arise a problem suchthat, when the vacuum from the vacuum source 2 b is applied to theholding surface 2 a of the chuck table 2 in a uniting step to beperformed later, the vacuum may leak from any gap between the polyolefinsheet 9 and the holding surface 2 a because the holding surface 2 a isnot fully covered with the polyolefin sheet 9, so that a pressure cannotbe properly applied to the polyolefin sheet 9.

In the processing method for the wafer 1 according to this preferredembodiment, a uniting step is next performed in such a manner that thepolyolefin sheet 9 is heated to unite the wafer 1 and the ring frame 7through the polyolefin sheet 9 by thermocompression bonding. FIG. 4 is aschematic perspective view depicting the uniting step according to thispreferred embodiment. As depicted in FIG. 4, the polyolefin sheet 9transparent or translucent to visible light is provided so as to fullycover the wafer 1, the ring frame 7, and the holding surface 2 a of thechuck table 2, which are all depicted by broken lines in FIG. 4. In theuniting step, the selector 2 c is operated to establish the ON conditionwhere the vacuum source 2 b is in communication with the porous memberof the chuck table 2, i.e., the holding surface 2 a of the chuck table2, so that a vacuum produced by the vacuum source 2 b is applied to thepolyolefin sheet 9 provided on the chuck table 2. Accordingly, thepolyolefin sheet 9 is brought into close contact with the wafer 1 andthe ring frame 7 by the atmospheric pressure applied to the uppersurface of the polyolefin sheet 9.

Thereafter, the polyolefin sheet 9 is heated in a condition where thepolyolefin sheet 9 is sucked by the vacuum source 2 b, therebyperforming thermocompression bonding. In this preferred embodimentdepicted in FIG. 4, for example, the heating of the polyolefin sheet 9is effected by a heat gun 4 provided above the chuck table 2. The heatgun 4 includes heating means such as a heating wire and an air blowingmechanism such as a fan. Accordingly, the heat gun 4 can heat ambientair and blow the heated air. In a condition where the vacuum from thevacuum source 2 b is applied to the polyolefin sheet 9, the heat gun 4is operated to supply hot air 4 a to the upper surface of the polyolefinsheet 9. Accordingly, when the polyolefin sheet 9 is heated to apredetermined temperature, the polyolefin sheet 9 is bonded to the wafer1 and the ring frame 7 by thermocompression bonding.

Another method for heating the polyolefin sheet 9 may be adopted. Forexample, any member heated to a predetermined temperature may be pressedon the polyolefin sheet 9 against the wafer 1 and the ring frame 7. FIG.5 is a schematic perspective view depicting such a modification of theuniting step. As depicted in FIG. 5, the polyolefin sheet 9 transparentor translucent to visible light is provided so as to fully cover thewafer 1, the ring frame 7, and the holding surface 2 a of the chucktable 2, which are all depicted by broken lines in FIG. 5. In thismodification depicted in FIG. 5, a heat roller 6 including a heat sourceis used. More specifically, the vacuum produced by the vacuum source 2 bis first applied to the polyolefin sheet 9, so that the polyolefin sheet9 is brought into close contact with the wafer 1 and the ring frame 7 bythe atmospheric pressure applied to the upper surface of the polyolefinsheet 9.

Thereafter, the heat roller 6 is heated to a predetermined temperature,and next placed on the holding surface 2 a of the chuck table 2 at oneend lying on the outer circumference of the holding surface 2 a asdepicted in FIG. 5. Thereafter, the heat roller 6 is rotated about itsaxis to roll on the chuck table 2 through the polyolefin sheet 9 fromthe above one end to another end diametrically opposite to the above oneend. As a result, the polyolefin sheet 9 is bonded to the wafer 1 andthe ring frame 7 by thermocompression bonding. In the case that a forcefor pressing the polyolefin sheet 9 is applied by the heat roller 6, thethermocompression bonding is effected at a pressure higher thanatmospheric pressure. Preferably, a cylindrical surface of the heatroller 6 is coated with fluororesin. Further, the heat roller 6 may bereplaced by any iron-like pressure member having a flat base plate andcontaining a heat source. In this case, the pressure member is heated toa predetermined temperature to thereby provide a hot plate, which isnext pressed on the polyolefin sheet 9 held on the chuck table 2.

Still another method for heating the polyolefin sheet 9 may be adoptedin the following manner. FIG. 6 is a schematic perspective viewdepicting such another modification of the uniting step. As depicted inFIG. 6, the polyolefin sheet 9 transparent or translucent to visiblelight is provided so as to fully cover the wafer 1, the ring frame 7,and the holding surface 2 a of the chuck table 2, which are all depictedby broken lines in FIG. 6. In this modification depicted in FIG. 6, aninfrared lamp 8 is provided above the chuck table 2 to heat thepolyolefin sheet 9. The infrared lamp 8 can apply infrared light 8 ahaving an absorption wavelength to at least the material of thepolyolefin sheet 9. Also in the modification depicted in FIG. 6, thevacuum produced by the vacuum source 2 b is first applied to thepolyolefin sheet 9, so that the polyolefin sheet 9 is brought into closecontact with the wafer 1 and the ring frame 7 by the atmosphericpressure applied to the upper surface of the polyolefin sheet 9.Thereafter, the infrared lamp 8 is operated to apply the infrared light8 a to the polyolefin sheet 9, thereby heating the polyolefin sheet 9.As a result, the polyolefin sheet 9 is bonded to the wafer 1 and thering frame 7 by thermocompression bonding.

When the polyolefin sheet 9 is heated to a temperature near its meltingpoint by performing any one of the above methods, the polyolefin sheet 9is bonded to the wafer 1 and the ring frame 7 by thermocompressionbonding. After bonding the polyolefin sheet 9, the selector 2 c isoperated to establish the OFF condition where the communication betweenthe porous member of the chuck table 2 and the vacuum source 2 b iscanceled. Accordingly, the suction holding by the chuck table 2 iscanceled.

Thereafter, the polyolefin sheet 9 is circularly cut along the outercircumference of the ring frame 7 to remove an unwanted peripheralportion of the polyolefin sheet 9. FIG. 7A is a schematic perspectiveview depicting a manner of cutting the polyolefin sheet 9. As depictedin FIG. 7A, a disc-shaped (annular) cutter 10 is used to cut thepolyolefin sheet 9. The cutter 10 has a central through hole 10 a inwhich a rotating shaft 10 b is fitted. Accordingly, the cutter 10 isrotatable about the axis of the rotating shaft 10 b. First, the cutter10 is positioned above the ring frame 7. At this time, the rotatingshaft 10 b is set so as to extend in the radial direction of the chucktable 2. Thereafter, the cutter 10 is lowered until the outercircumference (cutting edge) of the cutter 10 comes into contact withthe polyolefin sheet 9 placed on the ring frame 7. That is, thepolyolefin sheet 9 is caught between the cutter 10 and the ring frame 7,so that the polyolefin sheet 9 is cut by the cutter 10 to form a cutmark 9 a. Further, the cutter 10 is rolled on the polyolefin sheet 9along a circular line set between the inner circumference of the ringframe 7 (i.e., the periphery of the inside opening 7 a of the ring frame7) and the outer circumference of the ring frame 7, thereby circularlyforming the cut mark 9 a along the above circular line. As a result, apredetermined central portion of the polyolefin sheet 9 is surrounded bythe circular cut mark 9 a. Thereafter, a remaining peripheral portion ofthe polyolefin sheet 9 outside the circular cut mark 9 a is removed.That is, an unwanted peripheral portion of the polyolefin sheet 9including an outermost peripheral portion outside the outercircumference of the ring frame 7 can be removed.

The cutter 10 may be replaced by an ultrasonic cutter for cutting thepolyolefin sheet 9. Further, a vibration source for vibrating the cutter10 at a frequency in an ultrasonic band may be connected to the cutter10. Further, in cutting the polyolefin sheet 9, the polyolefin sheet 9may be cooled to be hardened in order to facilitate the cuttingoperation. By cutting the polyolefin sheet 9 as mentioned above, a frameunit 11 depicted in FIG. 7B is formed, in which the frame unit 11 iscomposed of the wafer 1, the ring frame 7, and the polyolefin sheet 9united together. That is, the wafer 1 and the ring frame 7 are unitedwith each other through the polyolefin sheet 9 to form the frame unit 11as depicted in FIG. 7B. FIG. 7B is a schematic perspective view of theframe unit 11 in a condition where the front side 1 a of the wafer 1 andthe front side 7 b of the ring frame 7 are exposed upward.

In performing the thermocompression bonding as mentioned above, thepolyolefin sheet 9 is heated preferably to a temperature lower than orequal to the melting point of the polyolefin sheet 9. If the heatingtemperature is higher than the melting point of the polyolefin sheet 9,there is a possibility that the polyolefin sheet 9 may be melted to suchan extent that the shape of the polyolefin sheet 9 cannot be maintained.Further, the polyolefin sheet 9 is heated preferably to a temperaturehigher than or equal to the softening point of the polyolefin sheet 9.If the heating temperature is lower than the softening point of thepolyolefin sheet 9, the thermocompression bonding cannot be properlyperformed. Accordingly, the polyolefin sheet 9 is heated preferably to atemperature higher than or equal to the softening point of thepolyolefin sheet 9 and lower than or equal to the melting point of thepolyolefin sheet 9. Further, there is a case that the softening point ofthe polyolefin sheet 9 may be unclear. To cope with such a case, inperforming the thermocompression bonding, the polyolefin sheet 9 isheated preferably to a temperature higher than or equal to a presettemperature and lower than or equal to the melting point of thepolyolefin sheet 9, the preset temperature being lower by 20° C. thanthe melting point of the polyolefin sheet 9.

In the case that the polyolefin sheet 9 is a polyethylene sheet, theheating temperature in the uniting step is preferably set in the rangeof 120° C. to 140° C. Further, in the case that the polyolefin sheet 9is a polypropylene sheet, the heating temperature in the uniting step ispreferably set in the range of 160° C. to 180° C. Further, in the casethat the polyolefin sheet 9 is a polystyrene sheet, the heatingtemperature in the uniting step is preferably set in the range of 220°C. to 240° C.

The heating temperature is defined herein as the temperature of thepolyolefin sheet 9 to be heated in performing the uniting step. As theheat sources included in the heat gun 4, the heat roller 6, and theinfrared lamp 8 mentioned above, some kind of heat source capable ofsetting an output temperature has been put into practical use. However,even when such a heat source is used to heat the polyolefin sheet 9, thetemperature of the polyolefin sheet 9 does not reach the outputtemperature set above in some case. To cope with such a case, the outputtemperature of the heat source may be set to a temperature higher thanthe melting point of the polyolefin sheet 9 in order to heat thepolyolefin sheet 9 to a predetermined temperature.

After performing the uniting step mentioned above, a dividing step isperformed in such a manner that the wafer 1 in the condition of theframe unit 11 is processed by a laser beam to form a plurality ofmodified layers inside the wafer 1 along the plural crossing divisionlines 3, thereby dividing the wafer 1 into individual device chips. Thedividing step is performed by using a laser processing apparatus 12depicted in FIG. 8A in this preferred embodiment. FIG. 8A is a schematicperspective view depicting the dividing step. FIG. 8B is a schematicsectional view depicting the dividing step. As depicted in FIG. 8A, thelaser processing apparatus 12 includes a laser processing unit 14 forapplying a laser beam 16 to the wafer 1 and a chuck table (not depicted)for holding the wafer 1. The laser processing unit 14 includes a laseroscillator (not depicted) for generating the laser beam 16 having atransmission wavelength to the wafer 1 (having a wavelengthtransmittable to the wafer 1). The chuck table has an upper surface as aholding surface for holding the wafer 1. The chuck table is movable in adirection parallel to the upper surface thereof, that is, movable in afeeding direction. The laser beam 16 generated from the laser oscillatorin the laser processing unit 14 is applied to the wafer 1 held on thechuck table. The laser processing unit 14 further includes a processinghead 14 a having a mechanism for positioning a focal point 14 b of thelaser beam 16 at a predetermined vertical position inside the wafer 1.

In performing laser processing to the wafer 1, the frame unit 11 isplaced on the chuck table in the condition where the front side 1 a ofthe wafer 1 is exposed upward. Accordingly, the wafer 1 is held throughthe polyolefin sheet 9 on the chuck table. Thereafter, the chuck tableis rotated to make the division lines 3 extending in the first directionon the front side 1 a of the wafer 1 parallel to a feeding direction inthe laser processing apparatus 12. Further, the chuck table and thelaser processing unit 14 are relatively moved to adjust a relativeposition, thereby positioning the processing head 14 a directly above anextension of a predetermined one of the division lines 3 extending inthe first direction. Thereafter, the focal point 14 b of the laser beam16 is positioned at a predetermined vertical position. Thereafter, thelaser beam 16 is applied from the laser processing unit 14 inside thewafer 1. At the same time, the chuck table and the laser processing unit14 are relatively moved in the feeding direction parallel to the uppersurface of the chuck table. Specifically, the focal point 14 b of thelaser beam 16 is positioned inside the wafer 1, and the laser beam 16 isapplied to the wafer 1 along the predetermined division lines 3. As aresult, the modified layer 3 a is formed inside the wafer 1. Note that,in FIG. 8A, the modified layer 3 a formed inside the wafer 1 is depictedwith broken lines. Accordingly, a division groove 3 a is formed in thewafer 1 along the predetermined division line 3 by the laser beam 16.The division groove 3 a has a depth from the front side 1 a to the backside 1 b of the wafer 1. In this dividing step, the laser beam 16 may beapplied under the following processing conditions. The followingprocessing conditions are merely illustrative.

Wavelength: 1064 nm

Repetition frequency: 50 kHz

Average power: 1 W

Feed speed: 200 mm/s

After forming the modified layer 3 a inside the wafer 1 along thepredetermined division line 3, the chuck table and the laser processingunit 14 are moved in an indexing direction perpendicular to the feedingdirection to similarly perform laser processing along the next divisionline 3 extending in the first direction. Thereafter, laser processing issimilarly performed along all of the other division lines 3 extending inthe first direction. Thus, a plurality of similar modified layers 3 aare formed along all of the division lines 3 extending in the firstdirection. Thereafter, the chuck table is rotated 90 degrees about itsvertical axis perpendicular to the holding surface thereof to similarlyperform laser processing along all of the division lines 3 extending inthe second direction perpendicular to the first direction. Thus, aplurality of similar modified layers 3 a are formed along all of thedivision lines 3 extending in the second direction.

When the laser beam 16 is focused inside the wafer 1 by the laserprocessing unit 14 to form the modified layer 3 a, leaked light of thelaser beam 16 reaches the polyolefin sheet 9 below the wafer 1. Forexample, in a case where the frame unit 11 adopts an adhesive tapeinstead of the polyolefin sheet 9, when the leaked light of the laserbeam 16 is applied to an adhesive layer of the adhesive tape, theadhesive layer of the adhesive tape is melted, so that part of theadhesive layer is fixed to the back side 1 b of the wafer 1. In thiscase, the part of the adhesive layer is left on the back side of each ofthe device chips formed by dividing the wafer 1. As a result, adegradation in quality of each device chip is caused. In contrast, inthe wafer processing method according to the present embodiment, thepolyolefin sheet 9 having no adhesive layer is used in the frame unit11. Accordingly, even if the leaked light of the laser beam 16 reachesthe polyolefin sheet 9, the adhesive layer is not fixed to the back side1 b of the wafer 1. Thus, quality of each device chip formed from thewafer 1 is favorably maintained.

Next, the polyolefin sheet 9 is expanded in a radially outwarddirection, so that the wafer 1 is divided into individual device chips.After performing the dividing step, a pickup step is performed to pickup each device chip from the polyolefin sheet 9. Expansion of thepolyolefin sheet 9 is performed by using a pickup apparatus 18 depictedin a lower portion of FIG. 9. FIG. 9 is a schematic perspective viewdepicting a manner of loading the frame unit 11 to the pickup apparatus18. As depicted in FIG. 9, the pickup apparatus 18 includes acylindrical drum 20 and a frame holding unit 22 having a frame supporttable 26 provided around the cylindrical drum 20. The cylindrical drum20 has an inner diameter larger than the diameter of the wafer 1 and anouter diameter smaller than the inner diameter of the ring frame 7 (thediameter of the inside opening 7 a). The frame support table 26 of theframe holding unit 22 is an annular table having a circular insideopening larger in diameter than the drum 20. That is, the frame supporttable 26 has an inner diameter larger than the outer diameter of thedrum 20. Further, the frame support table 26 has an outer diameterlarger than the outer diameter of the ring frame 7. The inner diameterof the frame support table 26 is substantially equal to the innerdiameter of the ring frame 7. The frame support table 26 has an uppersurface as a supporting surface for supporting the ring frame 7 thereonthrough the polyolefin sheet 9. Initially, the height of the uppersurface of the frame support table 26 is set equal to the height of theupper end of the drum 20 (see FIG. 10A). Further, the upper end portionof the drum 20 is surrounded by the inner circumference of the ringframe 7 in this initial stage.

A plurality of clamps 24 are provided on the outer circumference of theframe support table 26. Each clamp 24 functions to hold the ring frame 7supported on the frame support table 26. That is, when the ring frame 7of the frame unit 11 is placed through the polyolefin sheet 9 on theframe support table 26 and then held by each clamp 24, the frame unit 11can be fixed to the frame support table 26. The frame support table 26is supported by a plurality of rods 28 extending in a verticaldirection. That is, each rod 28 is connected at its upper end to thelower surface of the frame support table 26. An air cylinder 30 forvertically moving each rod 28 is connected to the lower end of each rod28. More specifically, the lower end of each rod 28 is connected to apiston (not depicted) movably accommodated in the air cylinder 30. Eachair cylinder 30 is supported to a disc-shaped base 32. That is, thelower end of each air cylinder 30 is connected to the upper surface ofthe disc-shaped base 32. Accordingly, when each air cylinder 30 isoperated in the initial stage, the frame support table 26 is loweredwith respect to the drum 20 fixed in position.

Further, a pushup mechanism 34 for pushing up each device chip supportedto the polyolefin sheet 9 is provided inside the drum 20. Further, acollet 36 (see FIG. 10B) capable of holding each device chip undersuction is provided above the drum 20. Both the pushup mechanism 34 andthe collet 36 are movable in a horizontal direction parallel to theupper surface of the frame support table 26. The collet 36 is connectedthrough a selector 36 b (see FIG. 10B) to a vacuum source 36 a (see FIG.10B).

In expanding the polyolefin sheet 9, each air cylinder 30 in the pickupapparatus 18 is first operated to adjust the height of the frame supporttable 26 such that the height of the upper end of the drum 20 becomesequal to the height of the upper surface of the frame support table 26.Thereafter, the frame unit 11 transferred from the laser processingapparatus 12 is placed on the drum 20 and the frame support table 26 inthe pickup apparatus 18 in a condition where the front side 1 a of thewafer 1 of the frame unit 11 is oriented upward. Thereafter, each clamp24 is operated to fix the ring frame 7 of the frame unit 11 to the uppersurface of the frame support table 26. FIG. 10A is a schematic sectionalview depicting a standby condition where the frame unit 11 is fixed tothe frame support table 26 set at the initial position. At this time,the plural modified layers 3 a have already been formed inside the wafer1 along the division lines 3.

Thereafter, each air cylinder 30 is operated to lower the frame supporttable 26 of the frame holding unit 22 with respect to the drum 20. As aresult, the polyolefin sheet 9 fixed to the frame holding unit 22 byeach clamp 24 is expanded radially outward as depicted in FIG. 10B. FIG.10B is a schematic sectional view depicting the expanded polyolefinsheet 9. When the polyolefin sheet 9 is expanded, a force toward aradially outward direction is applied to the wafer 1, and the wafer 1 isdivided with the modified layers 3 a as a start point, thereby formingindividual device chips 1 c. When the polyolefin sheet 9 is furtherexpanded, the spacing between any adjacent ones of the device chips 1 csupported to the polyolefin sheet 9 is increased as depicted in FIG.10B. Accordingly, each device chip 1 c can be easily picked up.

In the wafer processing method according to the present embodiment,after the wafer 1 is divided into individual device chips 1 c, a pickupstep of picking up the device chips 1 c from the polyolefin sheet 9 isperformed. In the pickup step, a target one of the device chips 1 c isdecided, and the pushup mechanism 34 is next moved to a positiondirectly below this target device chip 1 c as depicted in FIG. 10B.Furthermore, the collet 36 is also moved to a position directly abovethis target device chip 1 c as depicted in FIG. 10B. Thereafter, thepushup mechanism 34 is operated to push up the target device chip 1 cthrough the polyolefin sheet 9. Further, the selector 36 b is operatedto make the collet 36 communicate with the vacuum source 36 a. As aresult, the target device chip 1 c is held under suction by the collet36 and thereby picked up from the polyolefin sheet 9. Such a pickupoperation is similarly performed for all the other device chips 1 c.Thereafter, each device chip 1 c picked up is mounted on a predeterminedwiring substrate or the like for actual use.

In the case of forming the frame unit 11 by using an adhesive tape, theleaked light of the laser beam 16 applied to the wafer 1 reaches theadhesive tape in the dividing step, so that the adhesive layer of theadhesive tape is melted and fixed to the back side of each device chip.Accordingly, in this case, there is a problem that the adherence of theadhesive layer to each device chip causes a degradation in quality. Tothe contrary, in the wafer processing method according to this preferredembodiment, the frame unit 11 can be formed by using the polyolefinsheet 9 having no adhesive layer, in which the polyolefin sheet 9 isattached to the wafer 1 and the ring frame 7 by thermocompressionbonding. That is, an adhesive tape having an adhesive layer is notrequired. As a result, it is possible to prevent a degradation inquality of each device chip due to the adherence of the adhesive layerto the back side of each device chip.

The present invention is not limited to the above preferred embodiment,but various modifications may be made within the scope of the presentinvention. For example, while the polyolefin sheet 9 is selected from apolyethylene sheet, a polypropylene sheet, and a polystyrene sheet inthe above preferred embodiment, this is merely illustrative. That is,the polyolefin sheet usable in the present invention may be formed ofany other materials (polyolefins) such as a copolymer of propylene andethylene and an olefin elastomer.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer processing method for dividing a waferalong a plurality of division lines to obtain a plurality of individualdevice chips, the division lines being formed on a front side of thewafer to thereby define a plurality of separate regions where aplurality of devices are individually formed, the wafer processingmethod comprising: a ring frame preparing step of preparing a ring framehaving an inside opening for accommodating the wafer; a polyolefin sheetproviding step of positioning the wafer in the inside opening of thering frame and providing a polyolefin sheet having no adhesive layer ona back side or the front side of the wafer and on a back side of thering frame, such that the polyolefin sheet is in direct contact with thefront side of the wafer or the back side of the wafer and the back sideof the ring frame; a uniting step of heating the polyolefin sheet havingno adhesive layer while applying a pressure to the polyolefin sheetafter performing the polyolefin sheet providing step, thereby unitingthe wafer and the ring frame through the polyolefin sheet bythermocompression bonding to form a frame unit in a condition where thewafer and the ring frame are exposed; a dividing step of positioning afocal point of a laser beam inside the wafer, the laser beam having atransmission wavelength to the wafer, and applying the laser beam to thewafer along each division line, thereby forming a modified layer in thewafer along each division line to divide the wafer into the individualdevice chips, after performing the uniting step; and a pickup step ofpicking up each device chip from the polyolefin sheet after performingthe dividing step.
 2. The wafer processing method according to claim 1,wherein the uniting step includes a step of applying infrared light tothe polyolefin sheet, thereby performing the thermocompression bonding.3. The wafer processing method according to claim 1, wherein thepolyolefin sheet is larger in size than the ring frame, and the unitingstep includes an additional step of cutting the polyolefin sheet afterheating the polyolefin sheet, thereby removing a part of the polyolefinsheet outside an outer circumference of the ring frame.
 4. The waferprocessing method according to claim 1, wherein the pickup step includesa step of expanding the polyolefin sheet to thereby increase a spacingbetween any adjacent ones of the device chips, and each device chip ispushed up through the polyolefin sheet.
 5. The wafer processing methodaccording to claim 1, wherein the polyolefin sheet is formed of amaterial selected from the group consisting of polyethylene,polypropylene, and polystyrene.
 6. The wafer processing method accordingto claim 5, wherein the polyolefin sheet is formed of polyethylene, andthe polyolefin sheet is heated in the range of 120° C. to 140° C. in theuniting step.
 7. The wafer processing method according to claim 5,wherein the polyolefin sheet is formed of polypropylene, and thepolyolefin sheet is heated in the range of 160° C. to 180° C. in theuniting step.
 8. The wafer processing method according to claim 5,wherein the polyolefin sheet is formed of polystyrene, and thepolyolefin sheet is heated in the range of 220° C. to 240° C. in theuniting step.
 9. The wafer processing method according to claim 1,wherein the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.